1. Field of the Invention
The present invention relates to an image reading apparatus in which a read result is corrected, an image reading method and an image reading program.
2. Description of the Related Art
First, a structure of a mechanical system of a related art image reading apparatus will be described.
FIG. 14 is a block diagram showing an example of the structure of the mechanical system of the related art image reading apparatus. The mechanical system of the image reading apparatus includes a first carriage 2, a second carriage 3, a condensing lens 4, a one-dimensional image sensor device 5, an original document table glass 6, a platen cover 7, an illumination device 8, and mirrors 9, 10 and 11.
An original document is placed on the original document table glass 6 so that the original document writing surface faces the original document table glass 6 side, and the platen cover 7 is closed so that the original document is pressed to the original document table glass 6. When the original document is irradiated by lighting of the illumination device 8, reflected light from the original document is focused on the one-dimensional image sensor device 5 through the mirrors 9, 10 and 11 and the condensing lens 4.
The first carriage 2 including the illumination device 8 and the mirror 9 and the second carriage 3 including the mirrors 10 and 11 are moved by a not-shown carriage drive motor in synchronization with a read timing signal, so that sub-scanning of the original document is performed.
With respect to the original document, a pixel line is outputted at each read timing signal by the one-dimensional image sensor device 5 so that main scanning is performed, and the first carriage 2 and the second carriage 3 are moved so that the sub-scanning is performed. The intensity of the reflected light during the scanning is photoelectric converted by the one-dimensional image sensor device 5, and is outputted as an analog electric signal.
First, a structure of an electrical system of the related art image reading apparatus will be described.
FIG. 15 is a block diagram showing an example of the structure of the electrical system of the related art image reading apparatus. In this drawing, the same reference numeral as that of FIG. 14 denotes the same or equivalent part to that shown in FIG. 14, and its explanation will be omitted here. The electrical system of the image reading apparatus includes a one-dimensional image sensor device 5, an illumination device 8, an analog processing circuit 12, an A/D converter 13, a shading correction circuit 14, a page memory 15, an image processing circuit 16, a timing generation circuit 17, a CPU (Central Processing Unit) 18, a control panel 19, a memory 20, a mechanical system drive circuit 21, an illumination device control circuit 22, and an external interface circuit 23. The external interface circuit 23 is connected to an external apparatus.
The external apparatus is, for example, a storage apparatus such as a hard disk, a flash memory or an optical disk apparatus, a PC (Personal Computer), a printer or the like. Incidentally, the foregoing storage apparatus may be incorporated in the image reading apparatus.
The control panel 19 receives setting information from a user, and sends it to the CPU 18. The mechanical system drive circuit 21 is for driving the mechanism for sub-scanning, such as the first carriage 2 and the second carriage 3. The illumination device control circuit 22 controls the turning on and off of the illumination device 8.
The CPU 18 uses the memory 20 as a working memory, and operates in accordance with a program and data stored in the memory 20. Besides, the CPU 18 controls the timing generation circuit 17, the control panel 19, the mechanical system drive circuit 21, the illumination device control circuit 22, the shading correction circuit 14, and the external interface circuit 23. In accordance with instructions from the CPU 18, the timing generation circuit 19 sends timing signals, such as read timing signals, to the one-dimensional image sensor device 5, the analog processing circuit 12, the A/D converter 13, the shading correction circuit 14, and the page memory 15, and causes the respective parts to perform synchronous operations.
An analog electric signal of an image obtained by the one-dimensional image sensor device 5 is inputted to the analog processing circuit 12, and a pre-processing of A/D conversion is performed. Here, the analog processing circuit 12 includes a circuit to perform the pre-processing such as CDS (Correlated Double Sampling) or sample hold, analog electric signal amplification, or offset removal. The analog electric signal pre-processed by the analog processing circuit 12 is converted into a digital electric signal by the A/D converter 13, and is sent to the shading correction circuit 14.
The shading correction circuit 14 performs a shading correction processing on the digital electric signal to correct sensitivity variation between each main-scanning pixel in the one-dimensional image sensor device 5, uneven illumination of the illumination device 8 (especially in the main scanning direction) or the like, and stores the result thereof as correction data into the page memory 15. The shading correction circuit 14 may send the correction data directly to the image processing circuit 16.
The image processing circuit 16 performs an image processing according to an instruction received from the external interface circuit 23 on the correction data read out from the page memory 15, and sends output data as the result of the processing to the external apparatus through the external interface circuit 23. For example, in the case where the image reading apparatus is an image scanner, the image processing circuit 16 outputs the output data which the external PC or printer can use. Besides, for example, in the case where the image reading apparatus 1 is mounted in a copier, the image processing circuit 16 outputs the output data for driving an optical system so as to form a latent image on a photoconductive drum. The external interface circuit 23 receives the instruction of reading from the external apparatus, sends it to the image processing circuit 16, and sends the output data from the image processing circuit 16 to the external apparatus.
Next, the outline of an operation of image reading processing by the related art image reading apparatus will be described.
First, in the control panel 19, setting information is inputted by the user. The setting information is the setting of, for example, an operation mode (color mode, monochrome mode, etc.) in reading of an image, a reading resolution, a document size or the like. The setting information inputted to the control panel 19 is acquired by the CPU 18. The CPU 18 sends control signals to the respective parts based on the setting information. Next, when an image reading start instruction (start button) is inputted in the control panel 19, the CPU 18 starts the image reading processing.
FIG. 16 is a flowchart showing an example of the operation of the related art image reading processing. First, before reading of an original document image, the image reading apparatus performs a shading correction preparation processing to acquire reference data (black reference data and white reference data) used for a shading correction processing (S111 to S114). Here, a reading position (the first carriage 2) exists at an initial position. First, the CPU 18 turns off the illumination device 8 (S111). Next, the CPU 18 instructs the mechanical system drive circuit 21 to move the reading position from the initial position to a position just below a reference plate (first read object), causes the one-dimensional image sensor device 5 to read the reference plate in the turning-off state, acquires the read data for plural lines in the state where the reflectivity is close to 0 and makes it the black reference data (S112). Here, the reference plate is provided at, for example, a position adjacent to the original document position.
Next, the CPU 18 turns on the illumination device 8 (S113). Next, the CPU 18 instructs the mechanical system drive circuit 21 to move the reading position to the position just below the reference plate, causes the one-dimensional image sensor device 5 to read the reference plate in the turning-on state, acquires read data at known high reflectivity for plural lines and makes it the white reference data (S114).
Next, the image reading apparatus performs an original document image reading processing to read an original document image (second read object) (S115 to S116). Here, the CPU 18 instructs the mechanical system drive circuit 21 to move the read position to the original document position, the one-dimensional image sensor device 5 reads the original document, and the shading correction circuit 14 performs the shading correction processing on the image read by the shading correction circuit 14, so that the correction data is acquired (S115).
Here, the CPU 18 instructs the mechanical system drive circuit 21 to move the first carriage 2 and the second carriage 3, and when the first carriage 2 reaches the leading edge (left edge in the drawing) of the original document, the CPU 18 performs sub-scanning, and successively outputs the output data outputted from the image processing circuit 16 to the external apparatus through the external interface circuit 23. In the sub-scanning, the first carriage 2 and the second carriage 3 move from the left to the right in FIG. 14 at speeds of (speed of the first carriage 2):(speed of the second carriage 3)=2:1 so that the light path length always becomes constant.
Thereafter, the sub-scanning is continued, and when it reaches the trailing edge of the original document corresponding to the original document size in the setting information, the CPU 18 stops the output of the output data to the external apparatus, sends an instruction to reverse the moving direction of the first carriage 2 and the second carriage 3 to the mechanical system drive circuit 21, and sends an instruction to turn off the illumination device 8 to the illumination device control circuit 22. Here, in the case where the carriage drive motor is a pulse motor such as a stepping motor, the original document size is managed by the number of pulses, and the amount of movement is controlled.
When it is detected by a not-shown optical position sensor that the first carriage 2 and the second carriage 3 moving in the opposite direction to the sub-scanning reach the initial position, an instruction to stop the first carriage 2 and the second carriage 3 is sent to the mechanical system drive circuit 21, the read position is returned to the initial position (S116), and this flow is ended. In the case where the image reading start instruction is inputted in the control panel 19 during the image reading processing, the foregoing image reading processing is again executed after the image reading processing.
Next, the shading correction circuit 14 will be described.
FIG. 17 is a block diagram showing an example of a structure of the related art shading correction circuit. The shading correction circuit 14 includes a selector 141, an averaging circuit 142, a memory selector 143, a black reference storage memory 144, a white reference storage memory 145, and a shading correction arithmetic circuit 146.
In the foregoing processing S112, the selector 141 selects an output destination in accordance with the instruction from the CPU 18, and outputs the read data acquired by the one-dimensional image sensor device 5 to the averaging circuit 142. In order to reduce the influence of random noise, the averaging circuit 142 performs an averaging processing of output of the one-dimensional image sensor device 5 for plural lines. The memory selector 143 selects an output destination in accordance with the instruction from the CPU 18, and stores the black reference data subjected to the averaging processing into the black reference storage memory 144.
In the foregoing processing S114, similarly to the black reference data, the white reference data is subjected to the averaging processing by the averaging circuit 142. The memory selector 143 selects an output destination in accordance with the instruction from the CPU 18, and stores the white reference data subjected to the averaging processing into the white reference storage memory 145.
In the foregoing processing S115, the selector 141 selects an output destination in accordance with the instruction from the CPU 18, and outputs the original document read data as the read data of the original document image acquired by the one-dimensional image sensor device 5 to the shading correction arithmetic circuit 146. The shading correction arithmetic circuit 146 performs a processing to normalize (original document read data black reference data) by (white reference data−black reference data) with respect to each pixel in the main scanning direction. For example, in the case where correction data after the shading correction processing has a resolution of 8 bits, correction data IMG_SHD(n) of the nth pixel in the main scanning direction after the shading correction processing is expressed by the following expression:IMG—SHD(n)=255×(IMG(n)−ref_Black(n))/(ref_White(n)−ref_Black(n)).
Here, IMG(n) denotes the original document read data of the nth pixel in the main scanning direction, ref_Black(n) denotes the black reference data of the nth pixel in the main scanning direction, and ref_White(n) denotes the white reference data of the nth pixel in the main scanning direction.
FIG. 18 is a graph showing an effect of the related art shading correction processing. The horizontal axis indicates the pixel number in the main scanning direction, and the vertical axis indicates the luminance. The data indicates the original document read data before the shading correction processing and the correction data after the shading correction processing. In the original document image, the region of the pixel numbers 5000 to 5800 has a high density, and the other region has a low density. According to this drawing, it is understood that as compared with the original document read data before the shading correction processing, in the correction data after the shading correction processing, the sensitivity variation between each pixel in the main scanning direction in the one-dimensional image sensor device 5, and the uneven illumination of the illumination apparatus 8 are corrected.
Incidentally, as a related art technique relevant to the invention, there is an image reading apparatus in which plural light receiving element arrays are arranged in the sub-scanning direction, and the respective light receiving element arrays perform reading at different timings, so that the resolution in the sub-scanning direction is improved (see, for example, patent document 1).
However, in the case where the image reading apparatus includes plural light receiving element arrays as in the patent document 1, read data outputted from a specific light receiving element array is added with noise due to the influence of operation of another light receiving element array. Hereinafter, this noise is called cross-talk noise.
Patent Document 1
    JP-A-2004-289289